1. Field of the Invention
The present invention relates to the technical field of digital televisions and, more particularly, to a method and device for acquiring a channel with frequency offset less than half symbol rate particularly in but not limited to the Digital Video Broadcasting-Satellite (DVB-S) system.
2. Description of Related Art
In developing digital televisions, the first essential task is to establish a digital platform. In addition to pure hardware equipments such as a set-top box or television set, the digital platform needs to include digital channels and digital contents.
According to market segments, current global digital televisions are grouped and regulated by two main specifications: Advanced Television Systems Committee (ATSC) for American-based markets, and Digital Video Broadcasting (DVB) for Euro-based markets. The DVB systems can be further divided into DVB-C (Cable) for wired television applications, DVB-S (Satellite) for satellite television applications, and DVB-T (Terrestrial) for wireless television applications.
An official system provider can transmit a signal, such as DVB-S, over a satellite antenna disposed on a building through a satellite located on 22000 km and above. In this case, a quadrature phase shift keying (QPSK) is applied to transmit MPEG2 data, and all carrier frequencies and symbol rates associated with all channels of the system can be pre-stored in a receiver in order to rapidly receive the programs.
However, when the carrier frequency and symbol rate are updated, such a way is unable to receive the programs on the updated channel. Accordingly, for such a DVB-S receiver, a blind scan function is required in order to provide a simple operating mode to thereby allow a user to have an automatic program scan without knowing the programs received by a satellite in advance. Speed and accuracy are the most important factors in evaluating the performance of the blind scan. Rapid scan reduces the waiting time of the user.
A typical blind scan method is a frequency scan method controlled by a backend software. FIG. 1 is a block diagram of a typical receiver. As shown in FIG. 1, the operation principle is first to use the controller 11 for tuning the tuner 12 to a carrier frequency (CF). Next, a filter bank (not shown) and a timing recovery loop (not shown) in the baseband demodulation chip 13 are activated to operate at smallest possible symbol rate. The symbol rate is increased whenever loop does not converge. When the symbol rate is over a preset maximum and the loop cannot converge, it indicates that there is no signal at the frequency. In this case, the carrier frequency of the tuner is incremented by a pre-specified step size. Accordingly, the above process is repeated until the loop converges.
Apparently, that way has the disadvantage of very slow scan speed. Since the possible symbol rate may be diversified in a very wide range, such as from 1 M to 45 M bauds in DVB-S system, and a satellite signal occupies a quite wide spectrum, such as an input range from 950 MHz to 2150 MHz for a Ku/C band tuner, the combination produces numerous possibilities. Accordingly, it spends long time to complete a blind scan procedure. In addition, it takes much time to change the carrier frequency of the tuner, which further prolongs the scan time. Moreover, as the convergence of the timing recovery loop is quite slow, speed of loop-scanning the symbol rate by this method is quite limited. Therefore, the typical frequency scan method is extremely slow.
In practical situation, validness of the channel parameters may change with time. Channel acquisition based on the pre-stored parameters in the receiver may fail. Besides, as the carrier frequency and the symbol rate produced by the blind scan method is limited by the used algorithms and the carrier frequency step resolution of a tuner, the resulted carrier frequency offset (CFO) by the blind scan method can be large for channel acquisition. Hence, the carrier frequency and symbol rate obtained by the typical pre-store based or blind scan method requires a further recognition. Improvement on channel parameter acquisition is required.
Therefore, it is desirable to provide an improved simple method and device for acquiring a channel with frequency offset, so as to mitigate and/or obviate the aforementioned problems.